1. Field of the Invention
The present invention relates to a method of manufacturing a rare earth magnet.
2. Description of Related Art
Rare earth magnets made from rare earth elements such as lanthanoid are called permanent magnets and are used for driving motors of hybrid vehicles, electric vehicles, and the like as well as motors included in hard disks and MRIs.
As an index indicating magnet performance of these rare earth magnets, for example, remanent magnetization (remanent magnetic flux density) and coercive force may be used. Along with a decrease in the size of a motor and an increase in current density, the amount of heat generation increases, and thus the demand for high heat resistance has further increased in rare earth magnets to be used. Accordingly, one of the important research issues in this technical field is how to hold magnetic properties of a magnet when being used at a high temperature.
Examples of the rare earth magnets include commonly-used sintered magnets in which a grain size of crystal grains (main phase) constituting a structure thereof is about 3 μm to 5 μm; and nanocrystalline magnets in which crystal grains are refined into a nano grain size of about 50 nm to 300 nm. Among these, currently, nanocrystalline magnets have attracted attention because they can reduce the addition amount of expensive heavy rare earth elements while realizing the refinement of crystal grains or they do not need the addition of heavy rare earth element.
An example of a method of manufacturing a rare earth magnet will be briefly described. For example, a method of manufacturing a rare earth magnet (oriented magnet) is commonly used, this method including: rapidly solidifying Nd—Fe—B molten metal to obtain fine powder; press-forming the fine powder into a sintered compact; and performing hot deformation processing on this sintered compact so as to impart magnetic anisotropy thereto. Examples of the hot deformation processing include extrusion such as backward extrusion and forward extrusion; and upsetting (forging).
It is known that, during the hot deformation processing, oxygen contained in a magnet material deteriorates a Nd—Fe—B main phase, which causes a decrease in remanent magnetic flux density and coercive force. In addition, it is also known that, when a modified alloy is diffused in a grain boundary phase to recover the coercive force after the hot deformation processing, oxygen remaining in the grain boundary phase inhibits the infiltration of the modified alloy into the grain boundary phase.
On the other hand, regarding nitrogen contained in a magnet material, it is generally known that, when the oxygen content is suppressed, the nitrogen content is reduced along with oxygen, and an effect of the nitrogen content on the magnet material has yet to be actively studied.
Japanese Patent Application Publication No. 2013-89687 (JP 2013-89687 A) discloses a method of manufacturing a Nd—Fe—B rare-earth permanent magnet, the method including: dry-milling a magnet material in an atmosphere of a noble gas to obtain magnet powder; forming the magnet powder into a formed body in an atmosphere of a noble gas; and sintering the formed body at 800° C. to 1180° C., in which a remanent nitrogen concentration after sintering is 800 ppm or lower and more preferably 300 ppm or lower.
The manufacturing method disclosed in JP 2013-89687 A has a description regarding the nitrogen content, but the details thereof are not about an increase in nitrogen content for improving magnet performance but about suppression in the nitrogen content for improving the coercive force of the rare earth magnet.
In order for the manufactured rare earth magnet to have, high orientation, it is necessary to apply strong strains to a sintered compact during hot deformation processing. However, crystal orientation is disordered due to locally high stress generated during deformation, and this crystal orientation disorder causes a decrease in remanent magnetization.
The crystal orientation disorder under high stress during the hot deformation processing occurs for the following reasons. That is, typically, the hot deformation processing of a Nd—Fe—B rare earth magnet is performed by applying a stress of 100 MPa to 500 Mpa thereto in a temperature around 800° C. In this temperature range, a liquid phase (Nd-rich phase) appears in the grain boundary phase, and this liquid phase promotes the main phase (crystal) to rotate and move. However, due to the high stress which is applied to obtain high magnetic properties during the hot deformation processing, the liquid phase is pressed out, and a liquid-phase pool is locally formed. Due to this liquid-phase pool, an orientation alignment behavior such as rotation or movement of crystals is disturbed, which leads to orientation disorder of crystals around the liquid-phase pool.
Therefore, in order to reduce the liquid-phase pool, a method of reducing the applied stress during the hot deformation processing may be considered. However, in order to obtain high magnetic properties, it is necessary to apply a high stress. Therefore, a reduction in the applied stress is contradictory to the improvement of magnetic properties by the hot deformation processing. In addition, a magnet material is a brittle material and is likely to be cracked when being processed. Therefore, a process of reducing tensile stress is necessary in the hot deformation processing. For example, the application of high stress is inevitable during the above-described extrusion or upsetting (forging).